1. Field of the Invention
This invention relates to radio frequency (RF) receivers and more particularly to RF receivers for digital radio.
2. Description of the Related Art
Digital radio implementations include an approach that simulcasts a conventional frequency modulated (FM) signal with two digital sidebands approximately 100 kHz wide containing the digital radio information. Orthogonal frequency division multiplexing (OFDM) is used to provide digitally modulated subcarriers in the sidebands conveying the digital radio information. Another digital radio standard, Digital Audio Broadcast (DAB), uses OFDM without simulcast. One aspect of digital radio and OFDM in particular is that it is important to lock the receiver to the carrier frequency and to the symbol rate.
A block diagram of a typical low-IF receiver is shown in FIG. 1. A typical low-IF receiver 100 includes an analog RF front end (RFFE) 101 coupled to receive signals from an antenna 102, an analog to digital converter (ADC) 103, a digital front end (DFE) 105, and a baseband demodulator (BB) 106. The RFFE 101 typically includes a low noise amplifier (LNA) 109, mixers 111, programmable gain amplifiers 115, oscillators 117 and 118 and analog low-pass filters 119. The DFE 105 converts a low-IF or zero-IF digitized signal from ADC 103 into a baseband channelized signal sampled at a rate suitable for demodulation. Typical operational blocks of the DFE include digital frequency rotation 120 and a channel filter 121 to remove undesired out-of-band signals.
The RFFE shown in FIG. 1 has two voltage controlled oscillators (VCOs) 117 and 118, used to generate a mixer local oscillator (LO) signal and an ADC sampling clock, respectively. The architecture shown in FIG. 1 allows independent adjustment of the sampling and LO clocks by the BB 106. For digital demodulation the BB may adjust the ADC sampling clock in order to align the sampling instants to the transmitted symbols (symbol tracking loop).
For low power applications, e.g., for portable digital radio applications, improvements with respect to power, efficiency, and/or area are desirable.